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Water - why should
we drink 8 glasses of water per day?
By Maria Esposito, BSc (Hons)
Nutritional Therapist
Normally, approximately
2.5 litre of water is required each day for a fairly sedentary adult in
a normal environment to replace the total loss of water that occurs though
urine, faeces, skin and lungs (Naghii).
To replace the water
loss about 1 to 1.2 litre needs to be in drinks, (mainly water or cold herbal
teas), 0.9 ml is taken though food and 0.3ml will be made by the body (oxidative
metabolism) (William A. et al).
Effect of water restriction
- What happens if you don’t drink enough?
The following are a series
of changes in a case of water restriction:
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Increase in pulse
rate and in rectal temperature
-
Increase in respiration
-
Tingling and numbness
of fingers and feet
-
Increase in concentration
of blood
-
Diminution of blood
volume and more difficult circulation
The difficulty and inadequacy
of the circulation of blood under these conditions leads to:
-
difficult breathing
-
gastro-intestinal
upsets
-
nausea and appetite
failure
-
and eventually to
difficulty in muscular movements and emotional instability (Lloyd)
-
In an animal study,
where water was restricted the animals were highly irritable and in
some cases bad tempered (Lloyd)
Thirst
It has been shown that
under intense activity a man will voluntarily drink only about half as much
water as what is needed to replace his losses from sweating and urine. Only
after food and rest does a person crave the water needed to replace the
deficit. (Lloyd)
Herbal teas
Ingestion of hot drinks
may cause internal body temperature to rise, which will result in sweating
in order to cool the body down. Hot drinks are sometimes used in medical
practice when it is desired either to temporarily increase body temperature
of induce sweating (Lloyd). Therefore cold or cool drinks are preferred
to hot drinks to replace fluid during the day, especially if the hot drinks
are the only fluid daily intake.
In high intensity
exercise and in competitive sports
In a sport or activity
with duration of at least 30 minutes of continuous exercise, there is a
risk of impaired performance and even health damage due to hypo hydration
(low water). Sweating is usually the form accounting for most water loss
during exercise or high environmental temperatures. The major danger of
low fluid intake during a high performance is dehydration. This fluid deficit
can rapidly reach level that impede heat dissipation, reduce heat tolerance
and severely compromise cardiovascular function and exercise capacity. Prolonged
dehydration will lead to cell death and multiple cell losses can lead to
the person’s death (Naghii).
The need for replacement
of water will depend on the extent of the losses incurred during exercise.
Ingestion of plain water for a short duration (less than 90 minutes) is
enough to replace the water lost.
In high intensity activity
and competition ingestion of plain water in the post-exercise period results
in a rapid fall in the plasma sodium concentration and in plasma osmolality
(Maughan). The changes have the effect of reducing the stimulus to drink
(thirst) and of increasing urine output, both of which will delay rehydration.
Rehydration after the high activity can only be achieved if the sodium lost
in sweat is replaced as well as the water (Maughan). There is no need to
take any extra sodium tablets, unless dehydration is so severe, for most
individuals the normal dietary intake of minerals is adequate to maintain
sodium and electrolyte balance during repeated days of training (Wiliam
A). It is sufficient to eat a salty food or food high in natural salt.
Sports drinks typically
provide quantities of sodium, chloride and potassium (electrolytes), which
can be drunk if in high activity or in competing in a hot environment.
Sugary drinks
During physical activities
lasting less than 90 minutes, water is generally the beverage of choice,
however, the inclusion of small concentrations of carbohydrate (6%-8% carbohydrate)
have been shown to better sustain power output over water alone during physical
activities that produce fatigue in 60 minutes or longer (William A.).
References
Latzka William A. and
Montain Scott J (1999)’Water and electrolyte requirements for exercise’.
Clinics in Sports Medicine Volume 18 (number 3 issue)
Lloyd L.E., McDonald
B. E. and Crampton E.W. Fundamental of Nutrition (2nd edition)
(chapter 3 water & metabolism). W.H. Freeman & Company; San Franscisco
Maughan R. J. (1992)
‘Fluid Balance and Exercise’. Int. J. Sports Med. 13:S132-S135
Naghii M. R. (2000)
‘The significance of water in sport and weight control’. Nutrition and
Health: volume 14 pp. 127-132
Maria Esposito is
a qualified Nutritional Therapist BSc (Hons) and currently works at clinics
in Central and North London, including NatureWorks in Bond Street and Holmes
Place at Crouch End. You can contact her on 07956-662 954 or
esposito_m@yahoo.co.uk
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